In recent years, the standards of USB 2.0 and IEEE1394 are widely distributed as high-speed signal transmission interfaces and used in a large number of digital devices such as personal computers and digital cameras. These interfaces adopt the differential transmission method that transmits a differential signal by using a pair of signal lines to realize faster signal transmission than the conventional single end transmission method.
A common mode filter is widely used as a filter to remove noise on a high-speed differential transmission line. The common mode filter has characteristics that the impedance to a differential component of signals transmitted in a pair of signal lines is low and the impedance to a common mode component (common mode noise) is high. Therefore, by inserting a common mode filter between a pair of signal lines, common mode noise can be cut off without substantially attenuating a differential mode signal.
FIG. 12 is a schematic exploded perspective view showing an example of the structure of a conventional surface-mounted common mode filter.
As shown in FIG. 12, a conventional common mode filter 1 includes a thin-film coil layer 2 containing a pair of spiral conductors 5, 6 that are mutually electromagnetically coupled and magnetic substrates 3a, 3b provided above and below the thin-film coil layer 2 and made of ferrite. The thin-film coil layer 2 includes first to fourth insulating layers 2a to 2d stacked sequentially, a first spiral conductor 5 formed on the surface of the first insulating layer 2a, a second spiral conductor 6 formed on the surface of the second insulating layer 2b, and first and second lead conductors 8a, 8b formed on the surface of the third insulating layer 2c. 
An internal peripheral end 5a of the first spiral conductor 5 is connected to a first external terminal electrode 7a via a contact hole conductor 9a passing through the second and third insulating layers 2b, 2c and the first lead conductor 8a and an internal peripheral end 6a of the second spiral conductor 6 is connected to a third external terminal electrode 7c via a contact hole conductor 9b passing through the third insulating layers 2c and the second feeder conductor 8b. External peripheral ends 5b, 6b of the first and second spiral conductors 5, 6 are connected to external terminal electrodes 7b, 7d respectively. The external terminal electrodes 7a to 7d are formed on side faces and upper and lower surfaces of the magnetic substrates 3a, 3b. The external terminal electrodes 7a to 7d are normally formed by sputtering or plating of the surface of the magnetic substrates 3a, 3b. 
An opening 2h passing through the first to fourth insulating layers 2a to 2d is provided in a central region of the first to fourth insulating layers 2a to 2d and on an inner side of the first and second spiral conductors 5, 6 and a magnetic core 4 to form a magnetic circuit is formed inside the opening 2h. 
WO 2006/073029 discloses a terminal electrode structure of a common mode filter. The terminal electrode of the common mode filter has an Ag film formed by applying a conductive paste containing Ag to the surface of a component or by sputtering or vapor deposition and then a metal film of Ni is further formed by performing wet type electrolytic plating on the Ag film.
Japanese Patent Application Laid-Open No. 2007-53254 discloses a common mode choke coil having an outer shape of rectangular parallelepiped as a whole by successively forming an insulating layer, a coil layer containing a coil conductor, and an external electrode electrically connected to the coil conductor on a silicon substrate by thin-film formation technology. In the common mode choke coil, the external electrode is formed by extending on the upper surface (mounting surface) of the insulating layer. An internal electrode terminal is constituted as an electrode of a multi-layered structure in which a plurality of conductive layers is stacked.
The conventional common mode filter 1 shown in FIG. 12 has a structure in which the thin-film coil layer 2 is sandwiched between the two magnetic substrates 3a, 3b and thus has not only high magnetic properties and excellent high-frequency properties, but also high mechanical strength. However, the structure of the conventional common mode filter uses the upper and lower magnetic substrates 3a, 3b made of ferrite and a ferrite substrate is easy to break when thinned too much, making slimming-down of the substrate difficult. Further, the filter is made thicker by the two magnetic substrates 3a, 3b being stacked, which makes it difficult to provide as a lowered chip product. Moreover, a large amount of expensive magnetic materials is used, posing problems of high manufacturing costs and excessive specs of filter performance depending on uses.
Moreover, the conventional common mode filter 1 has the four micro external terminal electrodes 7a to 7d formed on the surface of individual chip components by sputtering or the like, posing a problem that it is very difficult to form the external terminal electrodes 7a to 7d with high precision. Further, the internal electrode terminal is formed of many stacked conductor layers in a common mode choke coil described in Japanese Patent Application Laid-Open No. 2007-53254 and thus, the probability of a failed electrode being formed is high and a problem of increased manufacturing costs due to an increase in man-hour for the electrode formation is caused.